In the field of chromatography, it is often necessary to measure impurities in a sample background that is different from the carrier gas used in the system.
Examples of such applications are the measurement of O2 in H2, CO in N2, etc. Many people involved in the art have designed methods or developed separation materials in the attempt of accomplishing such measurements.
A good explanation of the problems involved in such measurements, i.e. separation problems and the resulting detrimental effects on detectors, can be found in U.S. Pat. No. 5,360,467 which describes a method of separating and detecting impurities in using a fractional concentration detector. However, the method they suggest is quite complex to perform and no analytical results are reported.
Thus, the standard way to resolve these issues remains to use a method known as the heartcut method.
In fact, when the sample background is different from the carrier gas, the sample background may interfere with the impurity to be measured by overlapping or masking it. Furthermore, some chromatographic detectors may be overloaded and damaged by the sample background. In such application, the sample background must be first eliminated without affecting the impurities to be measured. The standard method to do this is the heartcut method.
FIG. 1 shows a typical analytical chromatographic system having a valve and column's configuration flowpath that path could be used to achieve this. Indeed, the illustrated heartcut system is provided with one sample loop, two valves V1, V2 and two separation columns. The valve V1 injects the sample loop volume into the first separation column. The function of the first column is to separate as much as possible the sample background from the impurities. The second valve V2 is then actuated in order to vent away the sample background gas eluting from the first column. Before the first peak of interest comes out of the first column, the valve V2 is restored to its original position in order to allow the gas existing the first column to flow into the second column, and then into the detector. In other words, the valve V2 is particularly actuated so as to open a window only for the peak of interest, which then flows into the second column. The second column's function, the analytical one, allows the separation of the impurities as individual peaks. Even with this particular two columns configuration, the sample background gas still produces a large tailing peak that dramatically limits the performance of the system in terms of sensitivity and repeatability. Moreover, in several typical applications, impurity levels are in ppm or ppb range while the sample background is almost 100% pure. Therefore, there is a large difference in the number of molecules between impurities and the sample background. Furthermore, when the elution time of the impurities to be measured comes just after the sample background gas, the standard heartcut method cannot conveniently work. A good example of such situation is the measurement of sub ppm of O2 in H2 background. In this case, even after the H2 heartcut, there is still too much H2 in the second column. FIG. 2 illustrates a typical chromatogram that is obtained with the system shown in FIG. 1. It is clear from FIG. 2, that the heartcut method cannot provide convenient results in this case.
Therefore, it would be desirable to provide an improved chromatographic system and an improved chromatographic method for measuring impurities in a gas sample that would overcome the above mentioned drawbacks of the prior art systems and methods. It would be even more desirable to provide a method that would advantageously allow extracting and measuring a peak of impurities masked by the sample background.